Shaded pole motor control



Jan. ll, 1949. M. L GREENOUGH 2,458,701

slum-:D IPOLE Morn coNTngL Filed Nov. 2o., 1945- Il 1J [Iv /ENTOR.

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Patented Jan. 1l, 1949 I l UNITED STATES PATENT oFElcE 2,458,701 SHADED POLE MOTOR CONTROL Maurice Leighton Greenough, Groveland, `Mass., rassignor to Radio Corporation of America, a corporation olf Delaware Application November 20, 1945, Serial No. 629,900 11 Claims. (Cl. S18-208) This invention relates generally to variable shading windings of the motor are connected in impedance electrical networks and more particuseries. Since the anodes of the two triodes have larly to an improved method of and means for opposite instantaneous phase, the loading oi' the varying electrically the impedance of a load cirshading windings will depen cult in response to an applied potential. 5 the control potential applied to the triode grid Numerous variable impedance load and control electrodes. If grid an circuits have been employed heretofore which the same polarity, the conductance of the triode utilize thermionic tubes as variable reactive or is a maximum value determined by the xed impedance network elements. The instant invenyD.C. bias. Grid and anode volta tion comprises a novel thermionic tube variable l lpolarity reduce the tube conductance', and hence impedance network wherein a diode and a triode reduce the loading on the corresponding shading are connected in parallel in opposite polarity and winding. Reduction of the loading of either circuit is that effectively full wave impedance l t at the motor will rotate in a direction and triode thermionic tube, no applied alternating control potential the Brieily in its simplest form, the network deshading windings are equally loaded with the scribed heretofore, may be employed for increasresult that the motor iield is in equilibrium. The ing or decreasing the effective loading of an D-C bias applied to the grid electrodes of the alternating potential circuit in response to the triodes is selected to provide the desired damping phase and magnitude of a second alternating of the motor shading windings. This damping is control potential. The load circuit provides an effectively removed when an alternating Control alternating potential which is applied through signal 1S applied tothe circuit.

the capacitor to the oppositely polarized ther- Accordingly one of the objects of the invention mionic tubes. A nxed D.C. bias voltage applied is to provide an improved method o maximum impedance of the load. The alternat- Work in response to the phase and magnitude of ing potentialapplied to the triode grid may beA 'fm applied alternating potential. Another object ing of a shaded-pole induction servomotor. A further object of the inventio a reversible shaded-pole induction servomotor motor in response to the phase and magnitude of whereby differential loading of two shading windan apphed potential. An additional object is ings permits the motor to be operated in opposite to provide an improved electrical circuit providand magnitude of a single applied alternating pedance in response to a single applied potencontrol potential. Briefly, the circuit comprises tial. A further object is to provide an improved wherein the thermionic tubes of each network ling Simultaneously the loading of two shading are energized from separate ones of the motor windings 0f a reversible Shaded-Dolo induction and are biased by a predetermined D.C. potenother object is to provide an impr controls, whereby the degree and direction of further object is to provide an rotation of the motor may be determined in remotor control circuit having hig spouse to the applied alternating potential. The with heavy damping as the improved servoh starting torque motor circuit appole induction motor. 3i,

preaches equilibrium. Another object is to provide en improved servomotor control circuit having high ioop-gain and great stability.

The invention wiii he described in greater detail by reference to the accompanying drawing of which Figure i is a schematic circuit diagram o a basic embodiment of the invention, and Figure 2 is a schematic circuit diagram oi a preerred embodiment ci the invention adapted to control a reversible shaded-pole induction servomotor. Similar reierence characters are applied to similar elements throughout the drawing,

Referring to igure i, potential, not shown, is connected to input terminais e, terminating the primary winding oi a 'transformer 'l having a secondary 'winding 9 coupled thereto. One terminal or" vthe secondary winding S is connected through a relatively large capacitor l i to the cathode of a diode thermionic tube i3 and to the anode of a triode thermionic tube I5. The remaining terminal of the secondary winding e is connected to the anode of the diode i3 and to the cathode of the triode i5. The control grid electrode of the triode it is connected to a source of D.C. bias potential such, for example, as a battery Il. A source of alterhating control potential, not shown, is connected to control input terminals i9 and 2l which are connected, respectively, to the bias battery l and to the cathode ofthe triode l5.

The diode I3 is conducting on negativehalf cycles of the alternating potential E developed across the secondary winding 9, while the triode I5 can conduct only on positive half cycles of said potential. Furthermore the conduction of the triode is determined'by the magnitude of the grid voltage. Due to the series capacitor lI there can be no direct current dowing through the secondary winding 9. Therefore, the uncontrolled diode current cannot exceed that of the controlled triode current. The bias voltage applied to the grid of the triode may be D.C., as illustratcd, or may be an alternating potential of the same frequency as the applied anode potential. In the latter instance a synchronous bias action results. If the control alternating potential applied to the terminals I9, 2l, is of the same instantaneous polarity to that of the potential E, applied to the triode anode, the conduction of the thermionic tubes vreaches a maximum value determined by the preselected bias voltage. A control potential of opposite instantaneous polarity reduces the conduction of the thermionic tubes to a minimum value. The conductance of the thermionic tubes determines the effective loading impedance Z which is effectively shunted across the secondary winding 9 and hence which is reflected to the input terminals It should be understood that a tetrode, pentode or other for thc triode Iii.

In the circuit of Figure 2, a reversible shadedof the type commonly employed in the servo systems, includes an input winding 33 connected to input terminals 35, 31. coupled to a source of alternating potential, not shown. The motor includes a squirrel cage rotor element 39 and a pair of shading windings 4I and 43 which are connected in series and coupled to the input winding 33. The common terminal 45 of thc shading windings 4I and 43 is connected to the anodes of a pair of diode thermionic tubes I3, I3', and to the cathodes of a pair of triode thermionlc tubes I5, i I5. The

a source o alternating multi-grid tube may be substituted remaining terminal oi the first shading winding il is connected through a first capacitor il to the cathode or the diode i@ and to the anode of the trlode it. Similarly, the remaining terminal of the second shading winding it is connected through a second capacitor Il to the cathode oi the diode i3 and to the anode oi the triode le whereby instantaneous potentials E and E', of opposite polarity as indicated by the arrows ll and are applied to the thermicnic tube anode-cathode circuits.

The control grid electrodes of the triodes it and i5 are connected together and are coupled through a source of ll-C1. bias potential il to the output of an amplitude and phase control iii. A source of alternating control potential is connected to control input terminals 53, 6b which are coupled through the amplitude and phase control El to actuate the grid circuits of the triodes It and lil'.

Each oi the shading winding circuits including the corresponding series capacitor and the shuntconnected diode and triode are substantially identical to the circuit described heretofore by reference to Figure i oi the drawing. Since the two shading winding circuits have opposite instantaneous phase, a control potential applied to the input terminals 53 and 55 will increase the loading upon one of the shading windings and simultaneously will decrease the loading upon the other of the shading windings. Thus the phase and magnitude of the applied alternating control potential will determine the direction and degree of rotation of the servomotor. The magnitude of the bias voltage source I' normally is adjusted to provide considerable loading on both shading windings in the absence of an alternating control .potential The equal loading of both shading windings provides relatively heavy damping of the motor which ls removed when an alternating control signal is applied'to the circuit. This feature results in improved motor stopping characteristics while starting torque is unimpaired.

An addditional feature of the circuit operation is that sudden changes in the control potential provide D.C. surges as the capacitors charge, thereby increasing the damping. Motor stopping time is further reduced, although some starting delay is encountered. With the preferred servomotor control circuit described, it has been found that-the system loop-gain may be increased as much as four or five times that of known servo control systems without self-oscillation of the servo control circuit.

Thus the invention disclosed comprises an improved electronic variable impedance and a highly efficient and stable servomotor control circuit providing full control of a reversible shadedpole induction servomotor having high electrical damping and high starting torque.

I claim as my invention:

1. An electronic variable impedance network including a rst source of alternating potential, 'a pair of thermionic tubes parallel connected in opposite polarity, a capacitor in series with said tubes, said tubes and said series capacitor being in shunt with said first source, a second source of alternating potential, unidirectional bias voltage means for said one of said tubes or establishing a loading control limit, and means connecting said second source to one .of said tubes to vary simultaneously the conductance of both of said tubes to vary the loading of said rst source.

2. Apparatus according to claim 1 including assegni means for adjusting the phase and magnitude of said second source of alternating potential to control said loading of said first source.

3. A control circuit for a shaded-pole induction servomotor having a shading winding including a iirst source ci alternating potential coupled to said motor and said winding, a pair of thermionic tubes parallel connected in opposite polarity, a capacitor in series with said tubes, said tubes and said series -capacitor being connected in shunt with said winding, a second source of alternating potential, unidirectional4 bias voltage means for said one of said tubes for establishing a loading control limit, and means connecting said second source to one of said tubes to vary simultaneously the' conductance of both of said tubes to vary the loading of said shading Winding and the rotation of said motor.

4. A control circuit for a reversible shaded-pole induction servornotor having a pair of seriallyconnected shading windings including a iirst source of alternating potential coupled to said motor and to said windings, two pairs of thermionic tubes, the tubes of each of said pairs being parallel connected in opposite polarity, capacitors in series with each of said windings and diiferent pairs of said tubes, a second source of alternating potential, and means connecting said second source of potential simultaneously to one `tube of each of said pairs of tubes to vary simultaneously the conductance of all of said tubes for varying in opposite sense the loading of said shading windings and for controlling the rotation of said motor.

5. Apparatus according to claim 3 including means for adjusting the phase and magnitude of said second source of alternating potential to control the degree of said motor rotation.

6. Apparatus according to claim 3 including bias voltage means for said one of said tubes for electrically damping said motor in the absence of an applied voltage from said second source.

7. Apparatus according to claim, 4 -including means for adjusting the phase and magnitude of said second source of alternating potential to control respectively the direction and degree of rotation of said motor. l

8. Apparatus according to claim 4 including bias voltage means for said one of said tubes of each of said pairs of tubes for electrically damping said motor in the absence of an applied voltage from said second source. I

9. A control circuit for a shaded-pole induction i servomotor having a shading winding including a. first sourcel of alternating potential coupled to said motor and said winding, a thermionlc diiii ode and a thermionic triode having their anodecathode circuits connected in opposite polarity, a capacitor in series with said anode-cathode circuits of said tubes, said tubes and said series capacitor being connected in shunt with the said winding, a second source of alternating potential, unidirectional bias voltage means for one of said tubes for establishing loading control limits, and means connecting said second source to the gridcathode circuit of said triade to varying simultaneousiy the conductance of both of said tubes to vary loading oi said shading winding and the rotation of said motor.

lil. A control circuit for a reversible shaded- 1d pole induction servomotor having a pair of serially-connected shading windings including a first source of alternating potential coupled to said motor and to said windings, two pairs of thermionic tubes each including a thermionic diode and triode, the tubes of each of said pairs having their anode-cathode circuits parallel connected in opposite polarity, capacitors in series with each of said windings and different pairs of said tubes, a second source of alternating potential, and

` means connecting said second source of potential simultaneously to the grid-cathode circuits of said triodes of each of said pairs of tubes to vary simultaneously the conductance of all of said tubes for varying in opposite sense the loading of said shading windings and for controlling the rotation of said motor.

l1. 'I'he method of utilizing a pair of themiionic tubes and a capacitor to provide a variable electronic impedance for a source of alternating potential comprising the steps of operating said tubes in shunt polarity opposition, applying said potential serially to said capacitor and said tubes, unidirectionally biasing one of said tubes for establishing an impedance control limit, and

40 varying the conductance of one of said tubes to vary the loading of said source.

MAURICE LEIGHTON GREENOUGH.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,109,776 Johnson Mar. 1, 1938 2,351,759 Grundmann June 20, 1944' FOREIGN PATENTS Number Country Date 587,525 Germany Nov. 4, 1933 

